llvmGen: Remove -fast-llvm flag
Issue #18076 drew my attention to the undocumented `-fast-llvm` flag for the LLVM code generator introduced in 22733532. Speaking to Moritz about this, the motivation for this flag was to avoid potential incompatibilities between LLVM and the assembler/linker toolchain by making LLVM responsible for machine-code generation. Unfortunately, this cannot possibly work: the LLVM backend's mangler performs a number of transforms on the assembler generated by LLVM that are necessary for correctness. These are currently: * mangling Haskell functions' symbol types to be `object` instead of `function` on ELF platforms (necessary for tables-next-to-code) * mangling AVX instructions to ensure that we don't assume alignment (which LLVM otherwise does) * mangling Darwin's subsections-via-symbols directives Given that these are all necessary I don't believe that we can support `-fast-llvm`. Let's rather remove it.
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